I noticed alot of people have problems with the Tsudo 4-2-1 header for the 2.2l Ecotec, I myself have tried to fix it so many times i almost just switched to Pacesetter. I noticed the last time that i took the header off that it seems to be the widest diameter right up at the flange.
I measured both inner diameters and also found that the ID of the header was 2" exact and the inlet ID of the down pipe was 2 1/8". I cut some pipes and used a lathe to taper the end and welded them on.
I welded all the way around both. The idea was for the new pipes to fit in the down pipe and almost eliminate the need for a gasket there in general. After grinding down a couple high spots on the welds i put the two together and it fit perfectly. I cut a new gasket to play it safe.
Everything seals great and and i have no leaks what so ever. I drove the car for a few hours and tightened the bolts again and did the same the very next day.